Rear Diff Gear Ratio Dilema
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Joined: Jan 2020
Posts: 61
Likes: 43
From: Northwest Indiana
Rear Diff Gear Ratio Dilema
Hello all. I have a 2001 coupe A4. According to the report code and the tag on the diff it is a 2.73 gear. My trans went out....I purchased a used unit with diff, 3.15 ratio.
I have the diablosport trinity tuner with a custom DIABLEW tune. I reached out to Lew, to see if he could provide a tune for me so I could install the 3.15 diff without any shift point issues.
The response I got back was a little concerning. He told me that all of my tune files indicate that the car was equipped with a 3.42 ratio diff and that the tune fes don't lie.
Has anyone run into anything like this? What do I go by the tune file or the tag on the diff? Is there an easy way to determine ratio with the diff out of the car and disconnected from the trans? Not sure how to move forward here. I have the old trans and diff out of the car and the new ready to go in but dont know whick diff to use.
Any help would be appreciated!
I have the diablosport trinity tuner with a custom DIABLEW tune. I reached out to Lew, to see if he could provide a tune for me so I could install the 3.15 diff without any shift point issues.
The response I got back was a little concerning. He told me that all of my tune files indicate that the car was equipped with a 3.42 ratio diff and that the tune fes don't lie.
Has anyone run into anything like this? What do I go by the tune file or the tag on the diff? Is there an easy way to determine ratio with the diff out of the car and disconnected from the trans? Not sure how to move forward here. I have the old trans and diff out of the car and the new ready to go in but dont know whick diff to use.
Any help would be appreciated!
Instructor
Joined: Oct 2018
Posts: 206
Likes: 47
From: southwest coast florida
The 3.42 ratio was ONLY for manual cars, 3.42 was the standard diff for manual...never in an auto
The automatic was only built with 2.73 standard and the performance axle option 3.15
Better tell your guy to bone up on C5 trannys....
my 2 cents without knowing your parameters.....stay with the 3.15 performance axle diff, this ratio offers a much more responsive tranny/ride. You want this with the automatic. I've owned both and frankly the 2.73 is lethargic, I just bought another '01 C5..A4 3.15 diff.
The automatic was only built with 2.73 standard and the performance axle option 3.15
Better tell your guy to bone up on C5 trannys....
my 2 cents without knowing your parameters.....stay with the 3.15 performance axle diff, this ratio offers a much more responsive tranny/ride. You want this with the automatic. I've owned both and frankly the 2.73 is lethargic, I just bought another '01 C5..A4 3.15 diff.
Last edited by bigthaw; Aug 24, 2022 at 04:08 PM.
Thread Starter
Advanced
Joined: Jan 2020
Posts: 61
Likes: 43
From: Northwest Indiana
The 3.42 ratio was ONLY for manual cars, 3.42 was the standard diff for manual...never in an auto
The automatic was only built with 2.73 standard and the performance axle option 3.15
Better tell your guy to bone up on C5 trannys....
my 2 cents without knowing your parameters.....stay with the 3.15 performance axle diff, this ratio offers a much more responsive tranny/ride. You want this with the automatic.
The automatic was only built with 2.73 standard and the performance axle option 3.15
Better tell your guy to bone up on C5 trannys....
my 2 cents without knowing your parameters.....stay with the 3.15 performance axle diff, this ratio offers a much more responsive tranny/ride. You want this with the automatic.
Thread Starter
Advanced
Joined: Jan 2020
Posts: 61
Likes: 43
From: Northwest Indiana
I was able to turn the input shaft on my original diff about 2.75 times to one revolution of the output shaft. So I am guessing that confirms that I do actually have a 2.73?
Safety Car
Joined: Mar 2005
Posts: 3,757
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From: Wisconsin
Safety Car
Joined: Jan 2012
Posts: 3,956
Likes: 520
From: Da U.P.
You're getting too hung up on what you have vs what the tune says. My M6 tune says in parts of the transmission segment that it's an A4 car. Don't sweat it too much, I'm sure between you and Lew you can figure something out. Just know that at least I have seen some weird stuff in the tune.
Thread Starter
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Joined: Jan 2020
Posts: 61
Likes: 43
From: Northwest Indiana
You're getting too hung up on what you have vs what the tune says. My M6 tune says in parts of the transmission segment that it's an A4 car. Don't sweat it too much, I'm sure between you and Lew you can figure something out. Just know that at least I have seen some weird stuff in the tune.
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Melting Slicks
Joined: Dec 2004
Posts: 2,847
Likes: 1,144
From: Sleepy Hollow- NJ
Not true/ accurate. Parts within the 2 series case are positioned differently but it can and has been done many times using a thicker ring or a machined spacer with standard gears.
DTE and RPM have done plenty 2 series gear swaps over the years with 3.42s, 3.73s, 3.90's and 4.10s.
DTE and RPM have done plenty 2 series gear swaps over the years with 3.42s, 3.73s, 3.90's and 4.10s.
Drifting
Joined: Sep 2009
Posts: 1,862
Likes: 594
From: Prescott, Arizona
Factory programming of the PCM is available for the 3.15/ A4 rear drive ratio, a dealer can retrieve that, but not for a higher (numerically) rear drive ratio such as a 3.73 which my A4 has.
Have you confirmed the actual ratio like you did for the original 2.73?
Have you confirmed the actual ratio like you did for the original 2.73?
Thread Starter
Advanced
Joined: Jan 2020
Posts: 61
Likes: 43
From: Northwest Indiana
I did and it is a 3.15. i finished the job and my tuner did send me a tune for that gear. Everything works perfectly. Thanks for everyone's input!
Racer
Joined: Oct 2021
Posts: 324
Likes: 157
From: Maine
Sorry to be late to the party but I installed 3.15 and when I got it tuned the guy called me and said it has 3.42 in the CPU.
So he changed it to 3.15 and said I was good to go.....................awesome. down the road I go and I hit redline and the rev limiter.
I go back and tell him that he need to change the shift tables not what's in the CPU. He take a day to educate himself and reduces the shift points by 15%.
I think he downloaded a program from someone on the Hp tuner forum.
Long story short it's all fixed and I am happy.
So he changed it to 3.15 and said I was good to go.....................awesome. down the road I go and I hit redline and the rev limiter.
I go back and tell him that he need to change the shift tables not what's in the CPU. He take a day to educate himself and reduces the shift points by 15%.
I think he downloaded a program from someone on the Hp tuner forum.
Long story short it's all fixed and I am happy.